Masaaki Ando

Hormonal control of the intestine and urinary bladder in teleost osmoregulation

Three varieties of osmoregulatory surface in teleosts are considered herein: the intestine, which is under bihormonal control (cortisol and prolactin); the urinary bladder, which is under unihormonal control (prolactin); and the gallbladder, which is not under hormonal control. Water movement in the isolated intestine of the eel increases during seawater adaptation, and cortisol is required to maintain the enhanced water absorption. This water movement is associated with the active transfer of ions, especially Cl ions. In freshwater eels, potential difference (PD) and short-circuit current (Isc) across the intestine were nearly zero or slightly serosa-negative, whereas in seawater eels and in freshwater eels injected with cortisol, the negativity of the PD and Isc increased. Development of a Cl− pump in seawater and cortisol-treated freshwater eels was further confirmed by ion-flux study. Prolactin injection into seawater eels decreased the rate of salt and water absorption in the gut. Prolactin seems to act on the gut surface as an antagonist to cortisol, by suppressing the Cl− pump and lowering permeability to water. In the isolated urinary bladder of flounders, water movement is reduced after transfer to fresh water or to dilute sea water. Prolactin injection into seawater flounders decreased osmotic permeability to water to the level of freshwater fish. This water movement is also associated with active transport of ions: ouabain inhibited both net influx of Na ions and water. In contrast to the eel intestine, there is no difference in net Na+ fluxes between freshwater and seawater flounder bladders; prolactin stimulates Na+ reabsorption. In sea water, the flounder bladder seems to absorb water from the urine by actively absorbing Na ions. In fresh water, the bladder seems to absorb mainly Na (and Cl) ions, and prolactin is involved in these changes by decreasing permeability to water and by stimulating the Na+ pump. On the other hand, the ion- and water-absorption capacity of the gallbladder isolated from the eel does not show any response to environmental salinity change nor to cortisol or prolactin treatment. It is suggested that cortisol may act primarily at the level of ion pumps, and prolactin primarily at the level of permeability barriers.

Active transport of chloride in eel intestine with special reference to sea water adaptation

Abstract 1. 1. The electrical potential difference (PD) and short-circuit current (Isc) across the intestine were near-zero in fresh water eels, and the active transport of Na+ and Cl− was almost identical. 2. 2. When eels were adapted to sea water, PD and (Isc) increased markedly in serosa-negativity against mucosa due to the development of active Cl− transport. 3. 3. The intestinal PD and Isc were serosa-positive in fresh water teleosts, while they were serosa-negative or near-zero in marine species. In rainbow trout, reversal of the PD was observed when they were transferred from fresh water to sea water. 4. 4. The electrical resistance of the intestine decreased during sea water adaptation of the eel and rainbow trout. 5. 5. The results are discussed in relation to a high rate of intestinal water absorption of teleosts in sea water.

Adaptive Changes in Ion and Water Transport Mechanism in the Eel Intestine

Living in aquatic environments with a wide range of salt concentration, teleost fishes are known to maintain their osmotic pressure and electrolyte concentrations at levels largely independent of the salinity of their environment. Freshwater teleosts, which are hyperosmotic to the environment, tend to gain water by osmosis. They drink little water and the kidneys have the task of removing excess water. In contrast, seawater teleosts constantly face a pressing problem in water conservation because of their hypertonic environment. In order to replace the osmotic loss of water, they drink surrounding sea water and absorb water with monovalent ions from the intestine. The excess sodium and chloride ions are extruded by the gill, leaving osmotically free water in the body1–4.

Hormonal control of drinking behavior in teleost fishes; insights from studies using eels

Marine teleost fishes drink environmental seawater to compensate for osmotic water loss, and the amount of water intake is precisely regulated to prevent dehydration or hypernatremia. Unlike terrestrial animals in which thirst motivates a series of drinking behaviors, aquatic fishes can drink environmental water by reflex swallowing without searching for water. Hormones are key effectors for the regulation of drinking. In particular, angiotensin II and atrial natriuretic peptide are likely candidates for physiological regulators because of their potent dipsogenic and antidipsogenic activities, respectively. In the eel, these hormones act on the area postrema in the medulla oblongata, a circumventricular structure without blood-brain barrier, which then regulates the activity of the glossopharyngeal-vagal motor complex. These motor neurons in the hindbrain innervate the upper esophageal sphincter muscle and other swallowing-related muscles in the pharynx and esophagus for regulation of drinking. Thus, the neural circuitry for drinking in fishes appears to be confined within the hindbrain. This simple mechanism is much different from that of terrestrial animals in which thirst sensation is induced through hormonal actions on the subfornical organ and organum vasculosum of the lamina terminalis that are located in the forebrain. It seems that the neural and hormonal mechanism that regulates drinking behavior has evolved from fishes depending on the availability of water in their natural habitats.